Related-art image forming apparatuses, such as copiers, facsimile machines, printers, or multifunction printers having at least one of copying, printing, scanning, and facsimile functions, typically form an image on a recording medium according to image data. Thus, for example, a charger uniformly charges a surface of an image bearing member; an optical scanner projects a light beam onto the charged surface of the image bearing member (i.e. photoconductive drum) to form an electrostatic latent image on the image bearing member according to the image data; a developing device supplies toner to the electrostatic latent image formed on the image bearing member to make the electrostatic latent image visible as a toner image; a transfer device transfers the toner image directly from the image bearing member onto a recording medium or indirectly from the image bearing member onto a recording medium via an intermediate transfer member; a cleaning device then cleans the surface of the image carrier after the toner image is transferred from the image carrier onto the recording medium; finally, a fixing device applies heat and pressure to the recording medium bearing the unfixed toner image to fix the unfixed toner image on the recording medium, thus forming the image on the recording medium.
Generally, the optical scanner is equipped with a rotary polygon mirror (hereinafter referred to as a polygon mirror) and optical devices such as an imaging lens. The polygon mirror and the optical devices are directly attached to a housing of the optical scanner. The optical devices are disposed near the polygon mirror. Such a known optical scanner forms the electrostatic latent image on the photoconductive drum serving as a latent image bearing member by deflecting and scanning the light using the polygon mirror. The imaging lens focuses the light onto the photoconductive drum.
During image formation, the polygon mirror rotates fast, generating heat from a shaft bearing of the polygon mirror. As a result, an area of the housing near the polygon mirror deforms due to thermal expansion, which results also in deformation of the housing to which the optical devices are attached, changing the orientation of the optical devices near the polygon mirror. A change in the orientation of optical devices causes an increase in spot diameter of the deflected light beams, thereby degrading imaging quality.
In the case of a tandem-type color image forming apparatus in which a plurality of photoconductive drums for different colors is arranged in tandem, a change in the orientation of the optical devices shifts a position of projection of light against the surfaces of photoconductive drums for different colors to a sub-scanning direction (a surface moving direction of the photoconductive drum), resulting in color drift.
To address such difficulty, in one approach, a surface of the housing of the optical scanner opposite the polygon mirror includes a hole, and the area of the housing near the polygon mirror thermally expands so as to fill the hole. In other words, the hole permits thermal expansion of the housing, thereby suppressing deformation of the housing near the polygon mirror and hence reducing changes in the orientation of the optical devices near the polygon mirror.
Although advantageous, while the hole can help reducing deformation of the housing due to thermal expansion, the fact that the housing itself deforms due to thermal expansion still remains. In other words, providing a hole is not enough to prevent deformation of the housing and changes in the orientation of the optical devices near the polygon mirror.